Electrical conductivity is an important property for certain categories of die attach adhesives within the field of electronic packaging and electronic component assembly. Electrical conductivity has traditionally been attained through the addition of electrically conductive fillers to liquid or film adhesive compositions. Examples of electrically conductive fillers include, for example, silver, copper, silver coated copper, gold, nickel, graphite, silver coated graphite, and the like. Frequently however, the conductivity of the compositions containing these conductive fillers is insufficient for many applications
In certain instances, it is desirable to render such thermosetting resin compositions conductive, either thermally or electrically. This is typically achieved by the addition of a conductive filler, typically a metallic filler, such as silver, in particle and/or flake form. While generally the addition of the conductive filler provides adequate conductivity to the composition, in certain instances greater conductivity is desirable. Such instances include those where a microelectronic assembler desires to validate its process prior to attaching the multitude of wire bonds from the semiconductor chip to the circuit board, and thus tests for electrical conductivity at the point where the chip is attached to the board. Other instances include those where the microelectronic assembler seeks to achieve a higher degree of thermal conductivity for thermal management or heat dissipation reasons.
In some cases, it may be possible to either increase the loading level of conductive filler, select a more conductive filler, or choose a combination of fillers or particle sizes of fillers that increases conductivity. Choosing a more conductive filler or a combination of fillers or particle sizes of fillers may be satisfactory for certain applications, but it would be desirable to simply maintain the selected conductive filler, and perhaps increase its loading level. However, increasing the loading level of the conductive filler may affect adversely the rheology of the composition, thereby causing dispensing and/or flow issues. Oftentimes, increasing the loading level of the conductive filler may even adversely affect the conductivity itself. It would be desirable to be able to confer a higher level of conductivity to a thermosetting resin composition, without having to adjust the identity or the loading of the conductive filler itself.
It has previously been reported that the addition of certain metal salts can be used to improve the electrical conductivity of metal filled adhesive compositions. However, the metal salts previously used to increase electrical conductivity are insoluble or display limited solubility in conductive adhesive formulations, such as formulations containing monomers and resins. Insolubility of the metal salt additives represents a major limitation for their practical use and manufacture. The use of conductivity promoters based on insoluble salts can, for example, lead to lot-to-lot performance variability, where the solubility to the salt is greater in one lot than that in another lot.
Performance will be affected by the practical limitations of particle size and dispersion control. A consequence of this problem is significant variation in electrical performance from batch to batch, and even changes in performance during use due to changes in solubility and instability of the salt. Indeed, this drift in performance was been recognized in US 2004/0225045 A1, (paragraph [0166]) through the statement that the “compositions do not have sufficient shelf stability under ambient temperature conditions to provide reproducible results”. It would thus be of interest to have conductivity promoters that operate under low temperatures compatible with the stability and reproducibility requirements of the electronics industry.